1. Field of the Invention
The invention relates to the field of electronic metering of variables such as the power consumption of a motor or other load. A controller module for a motor starter or contactor generates a pulse width modulated metering signal representing the level of power consumption by the load. The metering signal is coupled through an output stage having an opto-coupler, voltage regulator and comparator arranged to minimize switching times, and a current output transistor developing a 4 to 20 mA metering output signal.
2. Prior Art
Watt meters that sense the power line voltage and current supplied to loads are commonly used, for example, with integrating means operable to accumulate power usage. The sensing and integrating means can be electrical, mechanical, or a combination of the two. In industrial applications, average load current is a useful indication of power usage for purposes of automatic control. Accordingly, the level of power usage is encoded in an electric signal.
Sensing and control instrumentation for particular applications may encode information such as the level of a process variable via analog or digital signals, and it is frequently necessary to convert the information from one form to another to achieve certain objectives. Obtaining high accuracy or resolution of the encoded information is one concern. Other concerns include the number of conductors or signal lines needed to carry the information, the speed of the signalling path, its noise immunity, and the power requirements. Whereas digital encoding to a high resolution requires a high number of parallel or serial bits, the signal may be encoded as an analog current or voltage for signalling using a single conductor pair.
Monitoring and control instrumentation can be coupled to signals associated with inductive load circuits across optical couplers to protect the monitoring and control instrumentation from the influence of inductive surges and the like. The optical couplers provide an LED on the inductive side of the circuit and a phototransistor on the protected side, responsive to the LED. Optical couplers are not particularly high speed switching devices as compared to switching transistors. This is due to the large base-collector area of the phototransistor of the optical coupler. The switching time also is affected by the resistance and capacitance of the load circuit coupled to the output of the phototransistor, typically the collector of the phototransistor.
An optical coupler is a digital (on/off) device and is not conventionally used in an analog signal path. However, the opto-coupler can be operated in a manner that is digital but continuously variable like an analog signal. This is accomplished by varying the pulse width applied to the opto-coupler, i.e., by pulse width modulation. In that case, the switching time of the opto-coupler circuit is a critical consideration. To accurately reproduce the pulse width of the digital signal across the optical isolation barrier, the switching time of the optical coupler either must be very short, or must be precisely equal as to turn-on and turn-off time, regardless of variations between devices, variations in temperature, etc. Unless the switching times are very short compared to the pulse width variations, the optical coupler will introduce an error into the data encoded by the variable pulse width.
A number of control and interface circuits are available for signalling the levels of process variables via current signals. One standard specification is a 4 to 20 mA current output having an offset (4 mA) and a deflection proportional to the level of the process variable over an arbitrary "full" scale. The process variable may be encoded by an analog circuit at the sensing (transmitting) end, coupled to a monitoring and/or control system at the receiving end over a shielded twisted pair. Common mode noise is rejected at the receiving end, and the offset is subtracted. The deflection between zero and the arbitrary full scale is factored to recover the desired information, and often digitized, providing a numerical factor that is related to an actual measurement of the respective physical unit, e.g., wattage, temperature, pressure, displacement or other sensed physical parameter that is being monitored or controlled.
In the environment of the present invention, the average current consumption of a load coupled to an electrical contactor or motor starter is encoded in a current signal as described. Contactors or motor starters historically were wired to relay ladder networks to control starting, stopping, speed changes, reversing, etc. Typically, the contactor/starter circuits include circuit breakers for protection from overcurrent and/or undervoltage conditions. With the availability of inexpensive microprocessor chips, most of the logical functions of a relay ladder are provided by the processor, at substantially reduced expense and with much less circuit complexity.
The Westinghouse Electric Corporation ADVANTAGE.TM. line of electrical contactors provides a modular system of contactors, protective features and controls using microprocessors. A control module is coupled to one or more contactors for coupling the load to the power line in alternative configurations, for example wye or delta couplings of a three phase motor, and obviates the wiring associated with a relay ladder. The power consumption load on the motor can be approximated from the average current loading of the respective phases, which advantageously is determined numerically by a processor module coupled to the contactor.
In order to integrate the load into an industrial process or the like, the signal representing power consumption of the load must be transmitted to the monitoring or control instrumentation that advantageously is operated as a function of load power consumption. Both analog and digital aspects are involved in switching the contactor on and off, reporting alarms in the event of overload conditions or a trip due to overload, sensing current and voltage, determining the motor loading current, and reporting the data in a manner that is useful for interfacing to other monitoring/control devices. It would be advantageous to arrange such a combination of analog and digital aspects to obtain high resolution of the encoded information, a minimum of wiring and signal lines, minimal power requirements, good noise immunity, and high speed.